An apostrophe (‘) can be used to show that letters are missed out of a word (a contraction).
I am sure I didn’t pick up your pen.
(The two words did and not are joined and the apostrophe replaces the letter o in not.)
An apostrophe can also show ownership or possession. This means that something belongs to someone or something.
The boy’s pen disappeared during an English lesson.
James’ hat had a bobble on the top.
The children’s books were collected in by their teacher.
The girls’ bags were left in the changing rooms after PE.